1. Field of the Invention
The present invention relates to a film-forming apparatus and a film-forming method for forming a deposited film usable in the production of a semiconductor device such as a photovoltaic element including a solar cell by means of plasma chemical vapor deposition (hereinafter referred to as plasma CVD) and using an improved power application electrode capable of being stably maintained without suffering from deformation due to thermal expansion thereof, plasma radiation thereto, or film deposition thereon.
2. Related Background Art
As a typical example of an electronic device in which a photovoltaic element is used, there can be mentioned a solar cell which converts sunlight energy or other light energy into electric energy. Recently, public attention has focused on the power generation system using a solar cell since the solar cell has advantages such that it is safe, can be readily handled and that it can be used as a power generation source of providing clean energy without causing CO2 buildup.
Separately, there have proposed a variety of amorphous semiconductor materials capable of being used in the production of an electronic device including a solar cell. Of these amorphous semiconductor materials, attention has riveted to amorphous silicon (a-Si) semiconductor materials since they have advantages such that they can be readily formed in a thin film having a large area with a relatively large design freedom with respect to the composition, they can be readily controlled in a wide range with respect to their electric and optical characteristics, and therefore, they are suitable as constituents of various electronic devices including solar cells. Particularly, a film comprising such a-Si semiconductor material (hereinafter referred to as xe2x80x9ca-Si filmxe2x80x9d) is more advantageous in comparison with a film comprising a crystalline silicon semiconductor material (hereinafter referred to as xe2x80x9ccrystalline Si filmxe2x80x9d). That is, the a-Si film has an absorption coefficient against an energy near the energy distribution peak of sunlight which is greater than that of the crystalline Si film, the a-Si film can be formed at a lower film-forming temperature (substrate temperature) than that in the formation of the crystalline Si film, and the a-Si film can be directly formed on a given substrate from a silicon-containing raw material by way glow discharge. In view of this, the a-Si film has been generally recognized as being suitable as a constituent of a solar cell and in fact, it has been widely using in the production of a solar cell.
Now, for a solar cell which has been considered to be important as a part of the future energy measure, it is an immediate necessity in terms of the research and development to lower the production cost and to more improve the performance. In order to attain the production of a solar cell at a desirably low production cost, attention has riveted to an amorphous silicon (a-Si) semiconductor material capable of readily formed in a thin film state. Hitherto, there have been proposed various a-si semiconductor films having a relatively high photoelectric conversion efficiency in terms of the performance. However, these a-si semiconductor films are still insufficient in terms of lowering the production cost. As one of the reasons for this, there can be mentioned a subject in that their film-forming speed (their deposition rate) is slow. For instance, in the case of producing a p-i-n junction type solar cell by means of a glow discharge decomposition method, its i-type semiconductor layer is generally formed at a deposition rate of 0.1 to 2 xc3x85/sec which is relatively slow. In this case, in order to complete the formation of the i-type semiconductor layer having a thickness of 4000 xc3x85, it takes about 30 minutes to about 2 hours which is relatively a long period of time. As a method of forming such a-Si semiconductor layer having a relatively large thickness at a high deposition rate, there has been an attempt in that 100% SiH4 gas or 100% Si2Ho gas is used. Besides, Japanese Patent Publication No. 56850/1993 discloses that the deposition rate of a deposited film can be increased by shortening the distance between a power application electrode (a counter electrode) and a substrate capable of being served as an electrode.
Incidentally, in the case of forming a thin film of a semiconductor material (a semiconductor thin film) on a substrate using a film-forming apparatus by means of plasma CVD (this apparatus will be hereinafter referred to as xe2x80x9cplasma CVD film-forming apparatusxe2x80x9d) having a discharge space and a power application electrode, in order to make the semiconductor thin film formed on the substrate to have desired optical and electric properties, not only the substrate but also the discharge space including the power application electrode are heated to maintained at a given temperature. In addition, during the film formation, electrons and ions accelerated by the plasma discharge are collided with the substrate and the power application electrode to increase the temperature of the substrate and that of the power application electrode. In the case where the power application electrode comprises a single power application electrode, there is an occasion in that the power application electrode is thermally expanded due to the thermal energy caused by aforesaid electrons and ions to warp, bend or curve and as a result, the power application electrode is deformed to differ from the form of the power application electrode maintained at room temperature, where the distribution of the plasma generated becomes uneven. Separately, in the case where a semiconductor thin film is formed on a substrate using the foregoing plasma CVD film-forming apparatus, there is an occasion in that a film is deposited also on the power application electrode besides on the substrate and the power application electrode is deformed by virtue of a stress of the film deposited thereon.
Japanese Patent Publication No. 73327/1993 discloses a manner to prevent the occurrence of such problems in the plasma CVD film-forming apparatus. Particularly, in this document, there is described a contrivance in that the semiconductor electrode comprises a plurality of divisional electrodes arranged to have a relatively large interval between adjacent divisional electrodes which are electrically connected with a connector capable of varying their intervals, so that the distribution of the plasma generated is prevented from becoming uneven due to such deformation of the power application electrode as described in the above.
Now, it is considered that the deformation due to such thermal expansion and plasma radiation will occurred also at the substrate. However, as long as the substrate is fixed to a substrate holder, the substrate is not deformed. The substrate holder is usually provided with a heater and the like, and it has a size which is considerably larger than that of the substrate. Thus, the substrate holder is hardly deformed.
Separately, in the case where the substrate comprises an elongated web substrate which is transported to pass through one or more discharge spaces without being held by such substrate holder as above described, by fixing the web substrate by sucking its end portions by means of magnets or by virtue of the strength of a tensile force applied thereon, the web substrate can be prevented from being warped or deformed to a certain extent.
The influence which is given by such deformation of the power application electrode is small in the case where the distance between the substrate and the power application electrode is relatively small. However, in the case where the distance between the substrate and the power application electrode is shortened so as to increase the deposition rate of a film deposited on the substrate, there is an occasion in that even the power application electrode is deformed to a slight extent, such slight deformation of the power application electrode brings about a difference which cannot be disregarded in the distance between the substrate and the power application electrode, where unevenness is occurred in the distribution of the plasma generated to cause a partial difference for the film deposition rate.
An principal object of the present invention is to eliminate the foregoing problems in the prior art and to provide a film-forming apparatus by means of plasma CVD which has an improved power application electrode capable of stably maintained without suffering from deformation such as warping or curving due to thermal expansion thereof, plasma radiation thereto or film deposition thereon during film formation and a film-forming method by means of plasma CVD using said power application electrode.
Another object of the present invention is to provide a film-forming apparatus comprising at least a substantially enclosed vacuum chamber whose inside being capable of being evacuated, a power application electrode for introducing a discharging power into said vacuum chamber, and a raw material gas supply means for supplying a film-forming raw material gas into said vacuum chamber, said power application electrode being arranged in said vacuum chamber so as to oppose to a substrate arranged in said vacuum chamber. Wherein a prescribed film-forming raw material gas is supplied into said vacuum chamber through said raw material gas supply means and simultaneously with this, a prescribed discharging power is introduced into said vacuum chamber through said power application electrode to generate a plasma between said power application electrode and said substrate whereby decomposing said film-forming raw material gas to cause deposition of a film on said substrate, characterized in that said power application electrode has a reinforcing member for preventing said power application electrode from being deformed during the film formation.
A further object of the present invention is to provide a film-forming method comprising the steps of arranging a substrate on which a film is to be formed in a substantially enclosed vacuum chamber whose inside being capable of being evacuated of a film-forming apparatus having a power application electrode arranged in said vacuum chamber so as to oppose said substrate and a raw material gas supply means for supplying a film-forming raw material gas into said vacuum chamber, supplying a prescribed film-forming raw material gas into said vacuum chamber through said raw material gas supply means, and simultaneously with this, introducing a prescribed discharging power into said vacuum chamber through said power application electrode to generate a plasma between said power application electrode and said substrate whereby decomposing said film-forming raw material gas to cause deposition of a film on said substrate, wherein said power application electrode has a reinforcing member for preventing said power application electrode from being deformed during the film formation.
A further object of the present invention is to provide a film-forming apparatus comprising at least a substantially enclosed vacuum chamber whose inside being capable of being evacuated, a power application electrode for introducing a discharging power into said vacuum chamber, and a raw material gas supply means for supplying a film-forming raw material gas into said vacuum chamber, said power application electrode being arranged in said vacuum chamber so as to oppose to a substrate arranged in said vacuum chamber, wherein a prescribed film-forming raw material gas is supplied into said vacuum chamber through said raw material gas supply means and simultaneously with this, a prescribed discharging power is introduced into said vacuum chamber through said power application electrode to generate a plasma between said power application electrode and said substrate whereby decomposing said film-forming raw material gas to cause deposition of a film on said substrate, characterized in that said power application electrode has a thickness (d) which is greater than a distance (t) between said substrate and said power application electrode so that said power application electrode is prevented from being deformed during the film formation.
A further object of the present invention is to provide a film-forming method comprising the steps of arranging a substrate on which a film is to be formed in a substantially enclosed vacuum chamber whose inside being capable of being evacuated of a film-forming apparatus having a power application electrode arranged in said vacuum chamber so as to oppose said substrate and a raw material gas supply means for supplying a film-forming raw material gas into said vacuum chamber, supplying a prescribed film-forming raw material gas into said vacuum chamber through said raw material gas supply means, and simultaneously with this, introducing a prescribed discharging power into said vacuum chamber through said power application electrode to generate a plasma between said power application electrode and said substrate whereby decomposing said film-forming raw material gas to cause deposition of a film on said substrate, wherein said power application electrode has a thickness (d) which is greater than a distance (t) between said substrate and said power application electrode so that said power application electrode is prevented from being deformed during the film formation.